Large Terminase Protein (A) Enterobacteria Phage Lambda Recombinant Protein

Component of the terminase that processes and encapsidates viral genomes during virion assembly. The terminase lies at a unique vertex of the procapsid and is composed of two small subunits involved in viral DNA recognition and one large subunit possessing endonucleolytic and ATPase activities. To initiate packaging, it binds a specific sequence called cos, at the junction of adjacent viral genomes in the concatemeric DNA substrate. Next, in a reaction stimulated by the presence of proheads and ATP but not requiring ATP hydrolysis, the terminase creates two nicks 12bp appart at the cos site, one on each stand. Terminase then separates the cohesive ends in a reaction requiring ATP hydrolysis. The heterotrimer remains bound to the left end of the genome to be packaged, forming a stable DNA-protein complex known as complex I. In a reaction facilitated by a viral assembly catalyst, gpFI, complex I binds a prohead, a preformed head shell precursor, to form complex II. In another packaging reaction requiring ATP hydrolysis, the DNA is translocated into the prohead until the next cos site on the concatemer reaches the packaging complex. At this time the downstream cos site is cut and the heterotrimer undocks from the DNA-filled head to remain bound to the left end of concatemer's next genome. The new heterotrimer-DNA complex I binds another prohead to continue the processive, polarized packaging of viral genomes. The terminase is dependent upon host integration host factor (ihfA/ihfB) for these activities.